The need for optical-quality single crystals of materials exhibiting nonlinear optical properties is well established in the art. Potassium titanyl phosphate (i.e. KTP) is particularly useful in nonlinear optical devices, as described, for example, in U.S. Pat. No. 3,949,323. For many optical applications utilizing crystals optical-quality crystals having dimensions on the order of one millimeter or more are generally desired.
U.S. Pat. No. 3,949,323 discloses the preparation of crystals of compounds such as potassium titanyl phosphate by hydrothermal methods. Hydrothermal crystal growth processes have typically been high pressure processes involving crystal formation from a growth medium comprising mineralizer solution. A nutrient source such as a polycrystalline composition of the same material be provided, and seed crystals are often used to provide nucleation sites. Hydrothermal methods of crystal growth are considered particularly advantageous relative to flux methods of crystal growth for certain applications since they often produce crystals of relatively lower ionic conductivity. However, hydrothermal methods also typically require expensive reaction vessels to withstand the temperatures and/or pressures associated with crystal growth conditions and can involve crystal growth periods on the order of four to eight weeks to achieve crystals of the desired size and optical quality.
U.S. Pat. No 4,305,778 discloses a hydrothermal process for crystal growth of a group of materials including potassium titanyl phosphate which utilizes a mineralizer solution comprising at least in part a stable glass composition that minimizes the tendency of the seed crystals to dissolve in the aqueous mineralizer solution before nutrient can migrate to the seed crystals. Jia et al., "The Solubility of KTiOPO.sub.4 (KTP) in KF Aqueous Solution Under High Temperature and High Pressure", Journal of Crystal Growth, 79 (1986), pp. 970-973, discloses that by utilizing KF as a mineralizer relatively lower temperature and pressure can be employed for a hydrothermal KTP crystal growth process, and use of a pressure as low as 1000 Kg/cm.sup.2 (i.e. 14223 psi) is exemplified. Laudise et al., "Phase Relations, Solubility and Growth of Potassium Titanyl Phosphate, KTP", Journal of Crystal Growth 74,275 (1986) discusses various reactions relating to KTP as a function of temperature and KTP crystal growth at 10 Kpsi.
R. F. Belt et al. "Low Temperature Hydrothermal Growth of KTiOPO.sub.4 (KTP)" SPIG Proceedings 968 100 (1988) and U.S. Pat. No. 4,654,111 disclose hydrothermal processes for crystal growth of potassium titanyl phosphate, wherein favorable growth of potassium titanyl phosphate crystals can be maintained at relatively lower temperatures. Belt et al. discloses experiments where crystal growth is performed near 400.degree. C.; and presents a list of the main experimental conditions for high temeprature (590.degree. C., 25,000 psi) growth and low temperature (475.degree. C., 21,000 psi) growth. U.S. Pat. No. 4,654,111 discloses a process which achieves increased crystal growth rate at at relatively lower temperature ranges, e.g. 350.degree.-450.degree. C., but warns that the use of a potassium concentration in the mineralizer outside the range of 0.5 to 6.0 molar, calculated as elemental potassium, leads to poor quality crystal growth. The temperature reduction reportedly allows for the use of somewhat less expensive reaction vessels and, consequently, provides for a more economic process.
While substantial progress has been made in reducing the temperatures associated with hydrothermal crystal growth processes, the relatively high reaction pressures typically associated with the hydrothermal process can still limit the practical application of the process. For example, while lower temperatures allow the use of noble metal clad vessels for crystal growth, rather than containers within other containers, and the use of low carbon steel vessels, these less expensive reaction vessels may be limited in size due to the high pressures associated with this hydrothermal process.